An X-ray imaging system of this kind is known from U.S. Pat. No. 4,101,776.
The cited Patent Specification describes an X-ray imaging system comprising two control loops. The first control loop is an automatic dose control loop which adjusts the dose generated by the X-ray source so that the mean light intensity detected by the television pick-up device is constant. From the video signal generated by the television pick-up device the detection device determines a maximum value or a mean value of the video signal within a selected measurement field of the video image. This value is applied as a control signal to the power supply unit of the X-ray source and controls the cathode of the X-ray source. Thus, for objects which differ as regards X-ray absorption there are obtained video signals which have the same mean value or peak value. The video signal is applied to a television monitor via an amplifier which forms part of the second control loop, being an automatic gain control loop. The output signal of the amplifier is applied, for example to a further detection device which determines the mean value of the video image within a measurement field within the video image. The output signal of the further detection device is applied, for example to a comparator, one input of which is connected to a reference voltage. The output of the comparator is connected to the set terminal of the amplifier so, the gain factor thus being adjusted so that the mean level of the signal applied to the television monitor is constant. As a result of the use of an automatic gain control loop, the brightness of the image on the television monitor remains constant also if, in the case of very thick objects or objects exhibiting a high radiation absorption, the automatic dose control loop cannot increase the dose to such an extent that the mean light intensity on the entrance screen of the television pick-up device is sufficient. An upper limit as regards the dose to be adjusted by the dose control loop and to be generated by the X-ray source may be imposed by the fact that comparatively thick objects produce a substantial amount of scattered radiation, thus reducing the contrast in the X-ray image so that no contrast enhancement is achieved by increasing the dose. Furthermore, the maximum loadability of the X-ray source also imposes an upper limit as regards the dose that can be adjusted. Another advantage of an automatic dose control loop occurs in the case of dynamic imaging, i.e. imaging at the instant at which the X-ray source is activated or at the instant at which an object exhibiting local differences as regards X-ray absorption is moved within the X-ray beam. In those cases the automatic dose control loop readjusts the current and the high voltage of the X-ray source until the light intensity on the entrance screen of the television pick-up device reaches a predetermined constant value. When the dose increases upon activation of the X-ray source, the automatic gain control loop also increases the gain factor for the video signal applied to the television monitor, so that an image having the desired mean brightness appears on the television monitor already during the adjustment period of the automatic dose control loop. The time constants of the automatic gain control loop and the automatic dose control loop are approximately equal and are in the order of magnitude of from 100 to 200 ms, corresponding to 2.5 to 5 frame periods of a television image where a frame is formed by two interlaced fields. The fields are displayed on the television monitor with a frequency of 50 or 60 Hz. Because the X-ray dose and the gain factor for the video signal applied to the television monitor increase simultaneously, the video signal exhibits a peak which is perceived as a disturbing flash in the X-ray image.